Try reviewing the information in recover user data missing after Firefox update. It's likely that your data is still stored on your computer, but in a different profile.

I don't know whether it is explained in the above article, but the old profile folders and new profile folders are stored completely differently, so you do have to check both.

@t.r.ernst got it: this update actually changed my default profile, and all of the profiles *except* the previous default profile were appearing in the new Profiles menu, so it didn't occur to me that there could be more, hidden profiles accessible only through the backend. Thank you!